It is known that the bases of cooking vessels, in particular when they have a relatively small thickness of the order of 2 to 4 mm, for example, have a tendency to deform under the action of the heat source which is applied on the external face of said base.
This deformation consists of bulging-out of the base, the convexity of which is directed towards the heat source.
This bulging-out is due to the fact that, when cooking is in progress, the external face of the vessel base which is directed towards the heat is brought to a temperature which is distinctly higher than that of the internal face of the vessel.
The difference between these temperatures is increased by the presence of foods within the vessel which reduce the temperature of the internal face of this latter. Thus it commonly happens that the external face of the base is heated to over 500.degree. C. whereas the temperature of the internal face of the vessel does not exceed about one hundred degrees. As a consequence of this temperature difference, the metal expands to a greater extent in the zone adjacent to the external surface of the base than in the zone adjacent to the internal face. This accordingly results in bulging-out of the vessel base, the convexity of which is directed towards the heat source. This bulging-out is even more marked when the vessel is placed on a hotplate of cast-iron or of glass-ceramic material.
Bulging-out of the base of cooking vessels is subject to a number of disadvantages.
In the first place, bulging-out appreciably reduces the contact surface between the vessel base and the hotplate, thus resulting in waste of energy.
In the second place, bulging-out results in poor cooking conditions in view of the fact that fats and oils tend to accumulate at the center of the base instead of spreading uniformly over the internal surface of this latter.
In order to remedy the above-mentioned deformation, it has been proposed to employ cooking vessels having thick bases, that is to say bases having a thickness which attains the vicinity of 10 mm.
However, these thick-base vessels have the disadvantages of being costly, heavy to use, and of increasing cooking times since the thickness of these bases reduces the rate of heat transfer to foods.